The neurosympathetic responsiveness of two groups of uremics, each group treated for the same mean time by dialysis, categorized as normotensive, mean age 28.3 ± (SE) 3 years, or dialysis hypotension-prone, mean age 49.6 ± 5 years, was studied by determining plasma norepinephrine and epinephrine before and after postural activation of the sympathetic reflex arc. The study was performed on an interdialysis day and repeated 20 min before and after a regular dialysis on the following day. Standing caused norepinephrine to rise significantly in both the normotensive and the hypotension-prone groups and to a similar extent as in a control nonuremic group. Blood pressure did not change and the heart rate increased significantly. Plasma norepinephrine before dialysis 24 h later behaved in the same way, but both blood pressure and heart rate were unchanged. After dialysis, in spite of the usual increase in norepinephrine after standing, patients in the hypotension-prone group became hypotensive. These results suggest that there is a normal sympathetic responsiveness in uremics whatever their propensity for intradialysis hypotension. Moreover, dialysis does not seem to have an acute effect on the neurosympathetic response to a hypotensive stimulus. Dialysis-dependent hypotension, which appears to be more frequent in older subjects, probably reflects an impairment of the vasoconstrictor response of the vascular wall acutely induced by dialysis itsel